Conventionally, a cutting tool, which includes a cutting tool body and a hard-coating layer made of (a) a lower layer and (b) an upper layer, has been known. The lower layer of the cutting tool is a chemically deposited Ti compound layer composed of one or more of a titanium carbide (hereinafter referred as TiC) layer, a titanium nitride (hereinafter referred as TiN) layer, a titanium carbonitride (hereinafter referred as TiCN) layer, a titanium carboxide (hereinafter referred as TiCO) layer, and a titanium oxycarbonitride (hereinafter referred as TiCNO) layer. The upper layer of the cutting tool is a chemically deposited aluminum oxide layer (hereinafter referred as Al2O3). Also, conventionally, it has been known that the cutting tool described above can be utilized to a cutting operation of a wide variety of steel and cast iron.
However, chipping and fracturing are prone to occur in a cutting condition where a large load is subjected to its cutting edge in the above mentioned coated cutting tool. As a result, the cutting tool life is shortened. To circumvent this problem, several proposals have been made conventionally.
For example, in a coated cutting tool disclosed in Japanese Patent (Granted) Publication No. 4251990, an intermediate layer made of titanium boronitrilic oxide is provided between the lower and upper layers. By increasing the oxygen content in the intermediate layer from the lower layer side to the upper layer side, the bonding strength between the lower and upper layers of the hard-coating layer is improved. Thus, the chipping resistance of the coated cutting tool is improved.
In a coated cutting tool disclosed in Japanese Unexamined Patent Application, First Publication No. 2006-205300, a hard-coating layer, which has a titanium-based lower layer and an upper layer made of α-alumina layer, is proposed. In the coated cutting tool, titanium oxide portions are dispersively-distributed in a ratio of 1 to 50 parts in a range extending 10 μm in length from the interface between the lower and upper layers. Having the configurations, durability against impacts is improved, preventing the cutting tool from being chipped and fractured. As a result, a cutting tool, not only with excellent chipping resistance and fracturing resistance, but also with wear resistance, is provided. In a coated cutting tool disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-19603, a coated tool with a hard-coating layer including an upper layer. The upper layer is a porous Al2O3 layer having 5 to 30% of the porosity is proposed. Further a TiN layer is provided on the upper layer as a surface layer. Because of the above-mentioned configurations, thermal and mechanical impacts are absorbed and weakened. As a result, chipping resistance of the cutting tool is improved.